1. Field
Apparatuses consistent with exemplary embodiments relate to an optical device including a three-coupled quantum well structure, and more particularly, to an optical device including a three-coupled quantum well structure having multi-energy level, which may improve light absorption intensity in a multiple quantum well structure without increasing a driving voltage.
2. Description of the Related Art
3D cameras have not only a general image capturing function but also a function of measuring a distance from a plurality of points on a surface of an object to the 3D cameras. A variety of algorithms to measure the distance between an object and a 3D camera have recently been suggested. A typical one of these algorithms is a time-of-flight (TOF) algorithm. According to a TOF method, illumination light is emitted onto an object and then a flight time, between when the illumination light is emitted until the illumination light reflected from the object is received by a light-receiving unit, is measured. The flight time of illumination light may be obtained by measuring a phase delay of the illumination light. A high-speed optical modulator is used to accurately measure the phase delay.
An optical modulator having a superior electro-optical response characteristic is used to obtain a 3D image with high distance accuracy. Recently, GaAs-based semiconductor optical modulators are mainly used. A GaAs-based semiconductor optical modulator has a P-I-N diode structure in which a multiple quantum well (MQW) structure is disposed between a P-electrode and an N-electrode. In the structure, when a reverse bias voltage is applied between the P-N electrodes, the MQW structure forms excitons in a particular wavelength band and absorbs light. An absorption spectrum of the MQW structure characteristically moves toward a long wavelength as a reverse bias voltage increases. Accordingly, a degree of absorption at a particular wavelength may vary according to a change in the reverse bias voltage. Thus, according to the above principle, the intensity of incident light having a particular wavelength may be modulated by adjusting the reverse bias voltage applied to an optical modulator.
In the optical modulator, a distance accuracy increases as a contrast ratio, for example, a demodulation contrast, indicating a difference in the degree of absorption between when a voltage is applied and is not applied, increases. Driving at a low voltage is advantageous to prevent performance deterioration due to heat. In general, an increase in the contrast ratio may be achieved by increasing light absorption intensity and transition energy in the MQW structure. The light absorption intensity is inversely proportional to the thickness of a quantum well layer and is proportional to a square of a degree of superimposition between a hole's wave function and an electron's wave function in the quantum well layer. Also, transition energy that indicates a degree of an absorption spectrum moving toward a long wavelength is proportional to the fourth power of the thickness of a quantum well layer and to the square of an applied voltage.
However, when the thickness of a quantum well layer is reduced to increase the light absorption intensity, the transition energy decreases and an applied voltage increases in order to compensate for a decrease in the transition energy. On the other hand, when the thickness of a quantum well layer is increased to increase the transition energy, the degree of superimposition between a hole's wave function and an electron's wave function decreases and generation of excitons by electron-hole pairs is reduced so that absorption intensity decreases. Thus, the improvement of absorption intensity and the reduction of a drive voltage are in a trade-off relationship.